The present invention relates to imaging apparatus and methods, and more particularly relates to an imaging apparatus and method capable of ejecting liquid structures, which become spherical drops, within a short distance of travel from a nozzle orifice.
Inkjet imaging devices use the controlled ejection of small droplets of liquid, to produce an image. Typically, the liquid is ejected through one or more nozzle orifices, which are produced in a nozzle plate. The pressure pulse, which ejects the liquid drop through the nozzle orifice, is typically produced by the application of an electrical drive waveform to an electromechanical transducer, as in a piezoelectric printhead; or to an electrothermal transducer, or resistor, as in a thermal printhead. The present invention concerns electrical drive waveforms particularly designed for printing images requiring precise placement of the liquid drops on the receiving medium, as for example in graphic arts printing. In graphic arts printing the liquid drops may be deposited on plates which are then used to selectively attract ink that is transferred to an ultimate receiver sheet such as paper. Examples of ink or printing liquids used with lithographic printing plates are described in U.S. Pat. No. 6044762, however the invention is not limited to the fluids mentioned only in that patent but applies to other fluids suited for ejection from an inkjet printhead as taught herein which are generally referred to herein as an ink or printing liquid.
It is known to use specially designed electrical drive waveforms in inkjet printing, to achieve particular purposes. For reference example, Lee, et. al., U.S. Pat. No. 4,513,299 discloses a waveform comprising a series of pulses to eject a series of subdrops from a nozzle, which then merge prior to hitting the receiver surface, thus producing a liquid drop of variable volume. Paton, et. al., U.S. Pat. No. 5,361,084 also discloses ejecting a series of subdrops to achieve variable liquid volumes, from an array of nozzles. Burr, et. al., U.S. Pat. No. 5,495,270 discloses an electrical drive waveform technique in which higher order vibrational modes of the liquid meniscus are excited, in order to produce smaller liquid drops from a fixed nozzle size. Aoki, in U.S. Pat. No. 4,972,211 discloses the addition of a secondary pulse, added to the electrical drive waveform, to suppress residual pressure fluctuations at the meniscus, allowing higher drop firing rates.
However, none of the above references address the problem of forming spherical liquid drops at a spatial position close to the nozzle plate. It is accordingly an object of the present invention to provide a method for forming such liquid drops, in order to allow increased accuracy of the placement of the drops onto a receiving medium.
It has been known to use an inkjet printhead to eject drops of liquid onto the surface of a receiving medium to produce an image, as shown in FIG. 1. However, a problem with the prior art has been that in actual practice, the jet of liquid that is produced may emerge in a direction that is not exactly perpendicular to the surface of the nozzle plate, as shown schematically in FIG. 1, and in a real example in the stroboscopic photomicrograph of FIG. 2. The jet misdirection may arise from a variety of physical causes, such as nozzle imperfections or deposits, and it results in an error in the final location of the ink drop, or dot, on the receiver, with respect to its desired location. These location errors can cause artifacts in the resulting images, such as visible bands. It would be desirable to decrease the working distance, or the distance between the nozzle plate and the receiver, in order to reduce the dot placement error. However, as seen for example in FIG. 2, the liquid object which is actually ejected from a nozzle typically consists of a liquid droplet connected to a long ligament or tail. If a receiver in relative motion to the printhead were placed close to the nozzle plate, as for example at the head position of the droplet-tail object in FIG. 2, then a mark on the receiver would be formed in the shape of a comet, which is undesirable.
It is, therefore, an object of the present invention to provide a method of producing spherical liquid drops close to an ejecting nozzle, in order to achieve a short working distance, and improved dot placement accuracy, in an inkjet imaging apparatus.
An advantage of such a method is that images free of artifacts such as visible bands, may be produced. Another advantage of such a method is that images requiring high resolution and accurate dot placement, such as graphic arts images, may be produced.
In accordance with a first aspect of the invention there is provided a method of operating an inkjet printhead comprising providing an inkjet orifice of the printhead located within a predetermined spacing of less than 1000 micrometers from a receiver member that is moving relative to the orifice so as to present different portions of the receiver member to the orifice at the predetermined spacing; providing electrical drive signals to the printhead, the electrical drive signals being adapted to enable the printhead to generate a droplet of a printing liquid; and forming a free spherical droplet of the printing liquid between the orifice and the receiver member and depositing the droplet of the printing liquid upon the receiver member.
In accordance with a second aspect of the invention, there is provided an inkjet printing apparatus comprising a printhead having an inkjet orifice within a predetermined spacing of less than 1000 micrometers from a receiver member that is moving relative to the orifice so as to present different portions of the receiver member to the orifice at the predetermined spacing; and a source of electrical drive signals to the printhead, the electrical drive signals being adapted to enable the printhead to generate a free spherical droplet of a printing liquid substantially without presence of an attached or detached ligament of the printing liquid that would otherwise form a mark or artifact on the receiver.